Flexible foams are divided into two groups, i.e., foam comprising a polyester polyol (hereinafter referred to as "polyester foam") and foam comprising a polyether polyol (hereinafter referred to as "polyether foam"). The characteristics of these foams are drastically affected by the polyol used. Thus, these foams essentially differ in mechanical and chemical properties depending on the molecular structure of polyol, the intramolecular cohesive force of foam, etc.
The polyester foam has small cells, an excellent appearance and a great tensile strength and elongation. With respect to elongation after compression molding in particular, the polyester foam is very superior to the polyether foam. The polyester foam is also excellent in oil resistance and solvent resistance and chemically stable. Further, the polyester foam exhibits good heat resistance and weather resistance. However, the polyester foam has a great disadvantage that it shows a strength drop due to hydrolysis of ester bond attributed to polyester polyol used for the production of foam. The inhibition of hydrolysis will be hereinafter referred to as "resistance to wet heat aging". The polyester foam is further disadvantageous in that it suffers a relatively great hysteresis loss and thus is not suitable for use as cushioning material.
On the other hand, the polyether foam is very superior to the polyester foam with respect to hydrolysis. The polyether foam is also excellent in flexibility, restorability, etc. However, the polyether foam is inferior to the polyester foam with respect to oil resistance, solvent resistance, etc. Further, a polyether foam having a very low air permeability can be hardly obtained. Moreover, the polyether foam exhibits a small tensile strength and elongation. In particular, the polyether foam shows a great drop of strength and elongation after compression molding. Further, the polyether foam is considerably inferior to the polyester foam with respect to resistance to ozone deterioration. At present, therefore, the polyether foam yields to the polyester foam in many practical uses despite its advantage that it can be hardly hydrolyzed and exhibits an excellent resistance to wet heat aging. The inhibition of ozone deterioration will be hereinafter referred to as "resistance to ozone deterioration". Further, the polyether foam is inferior to the polyester foam with respect to resistance to deterioration by ultraviolet ray, etc. (The inhibition of deterioration by ultraviolet ray, etc. will be hereinafter referred to as "weather resistance".)
The flexible foam is essentially porous. Even when compressed under heating, the flexible foam cannot be provided with desired waterproofness regardless of whether it is of ester or ether type. In order to solve the problem of waterproofness, the following approaches have been proposed.
(i) A method which comprises providing a waterproof film on the surface of the flexible foam PA1 (ii) A method which comprises impregnating the flexible foam with an emulsion of a fluororesin, and then drying the flexible foam to remove water therefrom, thereby rendering the flexible foam water-repellent PA1 (i) The speaker edge must be so flexible as not to prevent free vibration of the cone paper; PA1 (ii) The speaker edge must be longitudinally flexible to retain the cone paper at the predetermined position, prevent the voice coil and magnet from coming in contact with the flame and maintain linear vibration but must be crosswise rigid to support the weight of the speaker cone and voice coil; and PA1 (iii) The speaker edge must have a low air permeability to sound-insulate the interior of the speaker box from the exterior, minimize the diffraction of sound, enhance the sound pressure at a bass range and improve the quality of the sound reproduced by the speaker. PA1 (i) Since this speaker edge is light and flexible, it can exhibit a maintained reproduction efficiency when applied to a small diameter speaker. Further, since this speaker edge produces little sound, a good quality reproduced sound having little noise can be obtained. PA1 (ii) This speaker edge can be three-dimensionally formed to cope with a wide range of amplitudes. PA1 (iii) The material of the speaker edge has an excellent stretchability itself. Thus, the resulting speaker exhibits little deterioration of sound quality due to edge creep when a large sound is reproduced. At the same time, the cone paper can follow the large output of sound at a bass range. Thus, a large sound can be produced from a speaker having a relatively small diameter. PA1 (iv) The hardness, strength and other properties of the speaker edge can be easily varied by controlling the compression factor during compression molding. By changing the compression factor, the lowest resonance frequency caused by the weight of various members and other variations can be controlled. PA1 (v) The raw material and the compression molding do not cost much.
However, the method (i) is disadvantageous in that it is difficult to form a homogeneous water-impermeable film on the surface of a flexible foam having numerous micropores. Further, as the material constituting the film there is often used a low heat-resistance material. A film formed of such a material can soften and melt to destruction when compressed under heating. Moreover, this method adds to cost. On the other hand, the method (ii) is disadvantageous in that it is considerably difficult to uniformly impregnate a flexible foam having the low air permeability with an emulsion of a fluororesin to enhance its waterproofness. Even if such a flexible foam can be uniformly impregnated with an emulsion of a fluororesin, it is difficult to completely remove water content from the foam, making it easy to render the waterproofness heterogeneous. Further, such an emulsion of a fluororesin is very expensive. Thus, the resulting foam, too, is very expensive.
It has been attempted to develop a foam having advantages of polyester foam and polyether foam in combination by the combined use of a polyester polyol and a polyether polyol. However, the two general-purpose polyols are poorly compatible with each other. Thus, the two polyols cannot be uniformly mixed and hence suffer phase separation that makes it impossible to provide a normal foam unless the mixing proportion of the polyester polyol is at least 50% by weight, particularly at least 80% by weight. The incorporation of a large amount of the polyester polyol makes it impossible to provide a foam well-balanced in physical properties. For the details of disadvantages caused by the mixing of a polyether polyol and a polyester polyol, reference can be made to Iwata, "Handbook of Polyurethane Resin", Nikkan Kogyo Shinbunsha, 1987, page 160. It is reported in this reference that the mixing of a slight amount of a polyether polyol in a polyester polyol during the preparation of a flexible polyurethane foam makes it impossible to obtain a normal cell, having a delicate effect on the cell conditions. Thus, when the foregoing two types of polyols are used in admixture to prepare a flexible urethane foam, the foam stability of which is important because its expansion rate is high, the resulting foam have cracks or voids. In some extreme cases, the foam can be destroyed, giving troubles in the production line.
It has been attempted to enhance the properties of a foam by the use of a polyether polyol having an ester bond incorporated therein. Further, a method which comprises using a special polyol such as phosphorus-containing polyester polyol in combination with a specific foam stabilizer and a method which comprises using a hydroxyl-terminated prepolymer or isocyanate-terminated prepolymer have been proposed. In a technical field of connecting a foam to a surface layer made of a polyvinyl chloride or the like by molding, the combined use of a specific polyester polyol and a polyether polyol has been proposed.
However, these proposed methods are conducted in restricted fields such as filter foam free of cell membrane (JP-B-52-35077 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-55-27315 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), etc.) and semirigid foam (JP-B-3-26694, JP-A-62-148516, etc.). These proposed methods are not put into practical use in the technical field of ordinary flexible foam. The preparation of the foregoing specific polyester polyol comprising a polyether polyol having an ester bond incorporated therein requires a complicated procedure. Thus, the resulting polyester polyol is expensive. If such a polyester polyol is used singly, the properties of the resulting foam cannot be widely varied. Further, the concentration of ester groups which can be incorporated into the foam is too low to provide a sufficient enhancement in the physical properties of the foam. U.S. Pat. No. 4,374,935 discloses that a mixture of a specific polyester polyol and a polyether polyol is used to improve hydrolysis resistance and mechanical properties such as compression strength of a flexible urethane foam. The specific polyester polyol can be obtained by the polycondensation of an organic dicarboxylic acid with a specific polyol mixture. However, this invention has excellent damping properties and thus is suitable for molding into automobile sheet. Further, this specific polyester polyol contains (or must contain) 1,4-butanediol during polycondensation. Thus, the resulting enhancement of hydrolysis resistance is limited to 5 days of exposure to 85.degree. C. moist atmosphere. No studies have been made of higher resistance to wet heat aging and resistance to ozone deterioration. With reference to application of sealing material, JP-B-2-55470 (filed by NHK Spring Co., Ltd.) discloses a process for the preparation of a flexible or semi-rigid open-cell type sealing material which comprises the use of at least one selected from the group consisting of dimeric acid polyol, castor oil polyol and polyether polyol obtained by the addition polymerization of at least 90 mol % of an alkylene oxide having at least 3 carbon atoms. However, no reference is specifically made to the mixing of a polyether polyol and a polyester polyol. Further, no studies are made of hydrolysis resistance (resistance to wet heat aging) or resistance to ozone deterioration.
A flexible foam finds wide application. In particular, a polyester foam has heretofore been often used as a material of speaker edge. A speaker comprises a cone paper, a voice coil connected to the cone paper for driving the cone paper, a magnet for driving the voice coil, and a flame for fixing the magnet. The speaker edge is a member which connects the cone paper to the flame in such an arrangement that the cone paper can be freely vibrated. At present, as such a membrane there is used a press-molded flexible foam, thermoformed resin-impregnated fabric or formed rubber or thermoplastic resin sheet.
The speaker edge has the following requirements:
As the material of the speaker edge there has heretofore been mainly used a polyester foam. This speaker edge can be normally obtained by cutting a sheet having a thickness of about 10 mm out of a flexible slab foam, and then compression-molding the sheet in a molding machine which has been controlled to a temperature of 200.degree. C. The speaker edge thus obtained is then connected to the cone paper and flame. The speaker edge made of a flexible foam meets the foregoing various requirements and has the following advantages as compared with speaker edges made of other materials.
In recent years, however, most automobiles have an on-board audio system. Thus, the recent speaker edge must have an even better durability. In particular, automobile doors having a speaker provided on its trim have been employed more and more. Thus, rainwater and other water contents can enter into the speaker through the gap between the door trim and the outer plate, resulting in an unexpected rise of humidity therein. Therefore, the flexible foam, which is essentially a porous material, can not form a speaker edge having a desired waterproofness even when thermoformed regardless of whether it is of ester or ether. It has been made clear that when a speaker edge made of polyester foam is used, its resistance to deterioration under wet heat becomes very important. Further, from the standpoint of weather resistance, even a speaker comprising a speaker edge made of a polyether foam cannot be used as an on-board speaker of the type provided on the rear tray, which can be easily irradiated with direct rays of the sun.